1. Field of the Invention
The present invention relates generally to an optical burst switching (OBS) network. More particularly, the present invention relates to an apparatus and method for adjusting a receiving time point of burst data and performing synchronization by compensating for a difference of the receiving time point of the burst data and a reference clock of a node which results from physical differences of links connecting nodes in an OBS network.
2. Description of the Related Art
Generally, an electric switch is used when an optical signal is transmitted through an optical fiber. In order to process the incoming optical signal. However, the electric switch should perform a process of converting the optical signal into an electric signal and then converting the electric signal into an optical signal. Accordingly, an opto-electric converter for converting the optical signal into the electric signal and an electro-optic converter for converting the electric signal into the optical signal are additionally required, and this causes the installation cost to increase.
In order to solve this problem, an optical burst switch that can directly process the optical signal without the necessity of converting the optical signal into the electric signal has been proposed. Hereinafter, an optical burst switching network using the optical burst switch will be explained.
Generally, in the optical burst switching network, internet protocol (IP) packets entering into an optical domain are gathered as burst data at an edge node, and such burst data are routed through a core node according to their destination or Quality of Service (QoS) to be sent to their destination node. Additionally, burst control packets (BCPs) and the burst data (BD) are offset and separately transmitted through different channels. That is, since the BCPs are transmitted prior to the burst data by an offset time and pre-engage a path through which the burst data is to be transferred, the burst data can rapidly be transmitted through the optical network. Hereinafter, the process of transmitting optical data will be explained with reference to FIG. 1.
FIG. 1 illustrates nodes that transmit and/or receive or switch the burst data in an optical burst switching network. The burst data is transmitted in the optical burst switching network as follows.
If IP packets are input, a node A 100 gathers the IP packets and makes burst data at an edge node. Edge nodes, such as node A 100, node D 106, or node E 108, serve to gather the IP packets, make and transmit a burst data packet, or to receive and separate an optical burst data packet into IP packets. Core nodes, such as node B 102 or node C 104, serve to perform optical switching of the optical burst data. If the burst data having a desired size is generated, the node A 100 generates and transmits the burst control packet (BCP) to the node B 102. After an offset time, the node A transmits the burst data to the node B 102. The BCP includes information about a destination address of the burst data, a source address of the burst data, a size of the burst data, a QoS, a offset time, or other control information known in the art.
The node B 102 confirms the destination address of the burst data to be received using the transferred BCP, determines an optical path, and reserves an optical switching time. Although an opto-electric or electro-optic conversion of the burst control packet is performed in the node B 102, the burst data follows the optical path through the optical switching without any opto-electric conversion. The node B 102 can perform optical switching of the burst data transmitted from the node A 100 according to the destination of the burst data, i.e., whether the destination of the burst data is the node D 106 or the node E 108.
As described above, the node B 102 transfers the burst data transmitted from the node A 100 to the node D 106 or the node E 108. However, the node B 102 may also be the destination of the burst data generated from the node A 100, or may directly generate the burst data to be transferred to the node D 106 or the node E 108. In other words, the node A 100 that is the core node may have the function of the edge node.
FIG. 2 illustrates links connected to a core node that includes a conventional optical switch. Referring to FIG. 2, the links connected to the core node are first to k-th input links and first to k-th output links. The burst data received through the first input link is output to one of the first to k-th output links by the optical switch 200.
As illustrated in FIG. 2, the burst data input to the optical switch 200 do not have the same input time point and size. In other words, the burst data arrive at the inputs to the optical switch at different times and the burst data are of different widths, i.e. one block of burst data may have more data than another. Since the size of the burst data is varied within a set maximum size, the size of the burst data transferred to the first input link is not generally equal to that of the burst data transferred to the k-th input link as illustrated in FIG. 2.
If the burst data input to the optical switch have different sizes and input time points, the switching efficiency of the optical switch deteriorates. Accordingly, in order to increase the switching efficiency of the optical switch, a scheme for equally setting the sizes and input time points of the burst data has been proposed, which is called a time-slotted optical burst switching (OBS).
FIG. 3 illustrates a time-slotted OBS. As illustrated in FIG. 3, the burst data transferred through the first to k-th input links connected to the optical switch 200 have the same size and input time point. By keeping the sizes and input time points of the burst data transferred to the links equal to one another as described above, the switching efficiency of the optical switch 200 can be heightened.
FIG. 4 is a view explaining a problem that may occur in the case of implementing the time-slotted OBS. In FIG. 4, a node A 400, a node B 402 and a node C 404 are illustrated. The node C 404 is connected to each of the node A 400 and the node B 402 by links, and receives the burst data from the node A 400 and the node B 402 using the links. Undoubtedly, the links transfer the burst control packet that includes control information about the burst data.
However, there may be physical differences among the links that connect the node A 400 and the node C 404, the link that connects the node B 402 and the node C 404, and the link connected to the output terminal of the node C 404. Referring to FIG. 4, the natural frequency of the link that connects the node A 400 and the node C 404 is fa=fnom+Δfa, and the natural frequency of the link that connects the node A 400 and the node B 402 is fb=fnom+Δfb. Also, the natural frequency of the link connected to the output terminal of the node C 404 is fc=fnom+Δfc. Accordingly, even if the node A 400 and the node B 402 transfer the burst data to the node C 404 through the links at the same time point, the arrival times of the burst data at the node C 404 become different from each other.
FIG. 5 illustrates the problem explained with reference to FIG. 4. As illustrated in FIG. 5, the receiving time points and time slot boundaries of the burst data transferred to the first input link, the burst data transferred to the second input link and the burst data transferred to the k-th input link are different from one another. Specifically, the receiving time point t1 of the burst data transferred to the first input link is relatively late in comparison to the receiving time point t2 of the burst data transferred to the second input link. Also, the receiving time point t2 of the burst data transferred to the second input link is relatively early in comparison to the receiving time point tk of the burst data transferred to the k-th input link. Accordingly, it is required to provide a method that can compensate for the differences among the receiving time points of the burst data and among the time slot boundaries that occur due to the differences among the natural frequencies of the links.